In data processing systems which use data terminals, a common operation is to enable both the main data processor and the terminal to cause certain operational function circuits to be activated or in the alternative to be turned off. With the great flexability of such systems, in the prior art, it is possible to have the main data processor terminate the operation of a certain function at a terminal although the user of the terminal in fact wants to continue to run that function. For instance, smooth scrolling, a data display device is such a function. Such a preemption of the terminal operation has often been at least a source of annoyance and in some cases has caused a loss of data. In the latter the data has to be regenerated. The present circuitry permits both the main data processor and the keyboard to turn the operational function circuits on and off, but also permits the keyboard to block the main data processor if the keyboard operator feels that the chosen operation at the terminal must continue.